can: kvaser_usb: Don't dereference skb after a netif_rx()

We should not touch the packet after a netif_rx: it might
get freed behind our back.

Suggested-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Ahmed S. Darwish <ahmed.darwish@valeo.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>